Digital transmission systems for communication usually include links in which many slower data streams are multiplexed together onto a single higher speed link. This higher speed link may be implemented by an optical transmission system. Such systems can be end to end links, or networks of optical links, with switching elements. Testing and verifying the elements of such an optical network can pose problems, particularly because the data rates are often very high.
It is known that connectivity through a network can be guaranteed by network layer protocols such as TCP/IP, which can request retransmission if some data does not arrive. At the physical data transfer level, path trace bytes can be inserted and checked to ensure a frame of data has been routed correctly. However, such methods involve demultiplexing the data stream, which only usually takes place at a terminal at the end of the path through the optical network.
Testing using analogue features is known, but only for limited purposes. It is known from U.S. Pat. No. 5,282,074 to obtain values of an optical signal before and after it passes through an optical amplifier. The values are subtracted to give a value for the noise introduced by the amplifier.
Other transmission systems in which analog signals are switched are known, such as old fashioned analogue telephone exchanges using relays. However, no attempt was made to verify connections, or compare input to output.
The present invention seeks to improve on such known arrangements.
According to one aspect of the invention, there is provided a verification system for switching element in an optical network comprising means for comparing an optical signal before it is input to the switching element, with an optical signal after being output from the switching element, and means for verifying from the comparison whether the switching element has switched the input signal correctly.
Such a system enables verification of each element individually, to aid fault isolation, without requiring expensive data demultiplexing hardware for each element.
A preferred feature involves using a portion of the frequency spectrum of the signal, for the comparison. This facilitates rapid comparison, and may reduce the expense or the quantity of the hardware required.
The feature of the input signal which is chosen for comparison may be unique, or if not unique, then at least partially independent from other input signals.
This independence enables verification of connectivity, and enables a degree of confidence in the verification to be established.
If the comparison means compares the input and output signals by pattern matching, the degree of confidence in the correctness of the verification can be improved.
A further advantageous feature involves verifying a correct connection on the basis of a threshold determined on the basis of a predetermined cost of false verification, and a predetermined cost of false non verification. This enables the threshold to be determined so as to optimize the verification for circumstances in which the cost or effect of a false verification is different from that of a false non-verification.
If the switching element comprises a plurality of input paths, the verification system may need to determine which input path is currently being switched to the output path, so that the corresponding signal is compared with the output signal by the comparison means. This enables a single comparison means to be used for multiple possible paths through the switching element, which enables a reduction in the amount and expense of the comparison circuitry.
Advantageously, the system comprises means for comparing input and output signals for a plurality of different inputs to the switching element, and determining a degree of cross-talk between said inputs.
This enables another parameter to be tested using essentially the same hardware as is used for the verification of connection.
Advantageously, where the switching element a plurality of input signals, there may be a means for comparing a pair of set plurality of input signals, and determining a degree of matching between patterns in them. This can assist in obtaining the value of cross-talk between inputs, and furthermore, it may assist in determining a degree of independence between input signals, which can be used to determine the confidence level in the correctness of the verification result. Advantageously, the threshold may be determined according to a desired confidence level.
According to another aspect of the invention, there is provided a signal analysis system for an element in a digital transmission system, comprising means for comparing an analogue feature of input signal of the element, with the same feature of a signal output from the element, wherein the comparing means is operable by pattern matching.
This enables a range of parameters of the element to be tested, or features of the element to be controlled, according to the result of the matching. By pattern matching features taken in analogue form from the signal passing through the element, rather than extracting digital features, the hardware can be simplified, and thus costs can be minimized. Advantageously, the element is a switching element, and the system comprises means for verifying from the matching result whether the switching element has switched the input signal correctly. This can be used either when setting up a new switching element, or for monitoring the performance of the element in service, to assist in detecting and isolating faults.
Advantageously, where the switching element has a plurality of input signals, the comparison means is arranged to compare the plurality of input signals with the output signal, and the system comprises means for determining the cross-talk between an input signal not switched to the output, and the output signal. This is one vital parameter of a switching element. Accordingly, fault isolation and determination of the cause of the fault can be facilitated if cross-talk can be determined.
Advantageously, the comparison means is arranged to compare the phase of a low frequency dither present of the input signal at a random phase, with a corresponding dither on the output signal. The use of a low frequency dither minimizes the disturbance to data traffic, while enabling detection with simpler hardware.
Another aspect of the invention provides a method of verifying the operation of a switching element in an optical transmission system, the method comprising the steps of: comparing an optical signal before it is input to the switching element, with an optical signal after being output from the switching element: and verifying from the comparison whether the switching element has switched the input signal correctly.
The invention will now be described by way of example, to show how it may be carried into effect, with reference to the drawings, in which;